494
13 Axial Compressors
on Mach number and turning. Modern optimisation techniques allow supercritical
cascades with a loss coefficient ω 1 as low as 0.015 [15].
Figure 13.19 is a sketch of the radial distribution of the Mach number in the
oncoming flow of rotors and stators within a three-stage transonic axial compressor
(LP-compressor of a bypass aero-engine; [6]). Stator cascades are supercritical near
the hub and subsonic near the casing. Rotor cascades are supercritical near the hub
and transonic near the casing. In compressor terminology, a cascade is transonic
when the inlet flow is supersonic and the outlet flow subsonic.
The design strategy of subsonic stator cascades near the casing is the same as
with supercritical cascades. A supercritical rotor cascade only differs from a supercritical stator by the higher Mach number level. With regard to shock strength, the
maximum Mach number at the suction side should not exceed about 1.25. This limitation is meant to prevent boundary layer separation due to the shock pressure rise.
For design conditions, blade shapes may be optimised with deceleration upstream
of the shock so that the Mach number just before the shock is close to one. But
away from the design operating point, the Mach number in front of the shock may
be much higher. The main effect of limiting the maximum Mach number is that the
optimum loading of a blade decreases with higher Mach number in the oncoming
flow and that solidity thus must increase.
13.4.2 Transonic Cascades
Figure 13.20 shows the shock pattern within a transonic cascade with low or medium high Mach number of the oncoming flow (up to Mach number about 1.2). Each
blade has a shock just upstream of the leading edge with an oblique part upstream
of the cascade, called the bow shock and a part normal to the adjacent blade surface,
Fig. 13.19 Variation of the
Mach number of the oncoming flow (rotor: R; stator:
S) as function of the radius
(normalised span) with a
three-stage axial compressor.
(Adapted to Fottner [6])
13 Axial Compressors
on Mach number and turning. Modern optimisation techniques allow supercritical
cascades with a loss coefficient ω 1 as low as 0.015 [15].
Figure 13.19 is a sketch of the radial distribution of the Mach number in the
oncoming flow of rotors and stators within a three-stage transonic axial compressor
(LP-compressor of a bypass aero-engine; [6]). Stator cascades are supercritical near
the hub and subsonic near the casing. Rotor cascades are supercritical near the hub
and transonic near the casing. In compressor terminology, a cascade is transonic
when the inlet flow is supersonic and the outlet flow subsonic.
The design strategy of subsonic stator cascades near the casing is the same as
with supercritical cascades. A supercritical rotor cascade only differs from a supercritical stator by the higher Mach number level. With regard to shock strength, the
maximum Mach number at the suction side should not exceed about 1.25. This limitation is meant to prevent boundary layer separation due to the shock pressure rise.
For design conditions, blade shapes may be optimised with deceleration upstream
of the shock so that the Mach number just before the shock is close to one. But
away from the design operating point, the Mach number in front of the shock may
be much higher. The main effect of limiting the maximum Mach number is that the
optimum loading of a blade decreases with higher Mach number in the oncoming
flow and that solidity thus must increase.
13.4.2 Transonic Cascades
Figure 13.20 shows the shock pattern within a transonic cascade with low or medium high Mach number of the oncoming flow (up to Mach number about 1.2). Each
blade has a shock just upstream of the leading edge with an oblique part upstream
of the cascade, called the bow shock and a part normal to the adjacent blade surface,
Fig. 13.19 Variation of the
Mach number of the oncoming flow (rotor: R; stator:
S) as function of the radius
(normalised span) with a
three-stage axial compressor.
(Adapted to Fottner [6])